Tire cords, especially, tire cords treated with an adhesive agent, referred to as “dip cords”, are widely used as reinforcing materials of rubber products such as tires, conveyor belts, V-belts and hoses and materials for tire cords include nylon fibers, polyester fibers, rayon fibers and the like. One of essential methods of improving performance of final rubber products is to improve physical properties of tire cords used as reinforcing materials.
Improved vehicle performance and road conditions have brought about gradually increasing vehicle driving speed. Accordingly, a great deal of research is underway on tire cords capable of maintaining stability and durability of tires even during high-speed driving.
A tire cord is classified depending on used part and rule and is divided into a carcass for entirely supporting the tire, a belt for supporting load upon high-speed driving and a cap ply for preventing deformation of the belt. Recently, improved highway conditions have resulted in increased vehicle speeds, causing problems such as deformation of the belt of the tire and deterioration in drive comfort. For this reason, importance of a cap ply to prevent deformation of the belt is increasing.
Major materials for currently used tire cords for cap ply are nylon and aramid. Of them, nylon is applied to most tires due to low cost, superior adhesivity and superior fatigue resistance as compared to other materials. In addition, nylon has a high compressive stress which is advantageous for preventing deformation of belts during high-speed driving. However, nylon has a problem of causing flat spots due to low modulus and great deformation with variation in temperature.
Aramid used as another material for the cap ply, in addition to the nylon, has almost no flat spot phenomenon in which a tire is deformed upon parking for a long time due to very high modulus and less change in modulus at room temperature and high temperature and is thus generally for high-quality tire essentially requiring high quality. However, aramid is inapplicable to general-purpose tires because it is very expensive. In addition, aramid has drawbacks of relative difficulty in tire molding due to high modulus, and low fatigue resistance and durability due to low breaking elongation.
In order to solve the aforementioned problems of nylon and aramid, a hybrid cord to which both nylon and aramid are applied has been developed. In particular, a hybrid cord having a structure in which a nylon primarily twisted yarn is covered with an aramid primarily twisted yarn (hereinafter, “covering structure”) has been developed.
Typically, in order to allow the aramid primarily twisted yarn and the nylon primarily twisted yarn to be simultaneously broken, the aramid filament yarn having a higher modulus is primarily twisted at a greater twist number as compared to the nylon filament yarn having a lower modulus and, in order to prevent aggregation between primarily twisted yarns during secondary twisting, the aramid filament yarn and the nylon filament yarn are primarily twisted in different directions. For example, an aramid primarily twisted yarn is produced by primarily twisting the aramid filament yarn at a high twist number in an S-direction, a nylon primarily twisted yarn is produced by primarily twisting the nylon filament yarn at a low twist number in a Z-direction, and a 2-ply yarn having a covering structure is produced by secondarily twisting the aramid primarily twisted yarn and the nylon primarily twisted yarn at a low twist number in an S-direction.
The 2-ply yarn having a covering structure described above has drawbacks of low production efficiency and high manufacturing cost because it is produced by a three-step process using a ring twister (that is, a first step of primarily twisting an aramid filament yarn to form an aramid primarily twisted yarn, a second step of primarily twisting a nylon filament yarn to form a nylon primarily twisted yarn, and a third step of secondarily twisting the aramid primarily twisted yarn and the nylon primarily twisted yarn).
In addition, there are problems of high deviation in physical properties and increased defect rate in the manufacture of the hybrid cord, because the aramid primarily twisted yarn for covering the nylon primarily twisted yarn is pulled by friction with a guide or roller to form a roof, or the nylon primarily twisted yarn is compressed, thus causing shape non-uniformity, during drying and thermal treatment after dipping the 2-ply yarn having a covering structure in an adhesive agent solution.
In addition, because the aramid filament yarn is primarily twisted at a higher twist number as compared to the nylon filament yarn in order to minimize the difference in physical properties between nylon and aramid, strength of the aramid filament yarn is greatly deteriorated and the advantage, i.e., high modulus, of the aramid cannot be secured. As a result, the hybrid cord having a covering structure inevitably has low strength as expected and thus has a relatively high risk of tire deformation during high-speed driving.
In order to solve the aforementioned disadvantages of the hybrid cord having a covering structure, Korean Patent Laid-open No. 10-2014-0090307, registered to the present applicant, suggests a hybrid cord having a merge structure which is produced by secondarily twisting a nylon primarily twisted yarn and an aramid primarily twisted yarn, which have been primarily twisted in the same direction, in a direction opposite to the direction, wherein secondary twisting is conducted such that the nylon and aramid primarily twisted yarns have an identical structure.
However, in the case of a hybrid cord having a merge structure, stress is intensely applied to the aramid primarily twisted yarn upon repeated tension and compression of tires, thus inevitably causing low fatigue resistance of tire cord and, disadvantageously, resulting in impossibility of securing safety of tires during long-term high-speed driving.